1. Field of the Invention
The present invention relates to a system and a method used for generating water vapor so as to provide a controlled humidity environment. More particularly, it relates to an apparatus and method which may have, among other applications, the control of humidity in a wood drying kiln.
2. Prior Art
Lumber drying kilns put 20,000 to 100,000 board feet (or more) of various species of lumber, with various dimensions in the kiln, and then heat the closed kiln to between 1000 and 1800 Fahrenheit (approximately 380 to 820 Celsius) for many days to extract moisture from wood. Thicker pieces of harder wood take longer to dry, sometimes up to 45 days (or longer). A major difficulty in drying wood is that the surface of the wood dries out more quickly than the center of a given piece. The result is checking, cracking, crazing or warping of the wood surface. Since surface clarity is the selling feature for cost of the dried wood, the value of the wood is greatly diminished or reduced to zero due to such checking, cracking, crazing or warping.
In general, the normal approach used in a wood drying kiln is to gradually increase the temperature, while controlling the moisture levels in the kiln using the differential between wet bulb and dry bulb temperature sensors.
The benefits of controlling humidity in the kiln include maintaining a given drying schedule and obtaining surfaces with no checking, cracking, crazing or warping, or shortening the processing or drying time. The higher temperatures allow shorter drying periods for reaching the desired 6 to 8% moisture levels of dried wood by maintaining the humidity at a higher level when higher temperatures are used.
A conventional approach for maintaining higher humidity is to use hot steam in the kiln. However, the use of steam often raises the dry bulb temperature of the kiln at too high a rate. Further many plants do not have sufficient steam capacity to provide the required amount of steam. Typically a plant has a given boiler, and over the course of many years has added several kilns. There is enough steam available to dry the wood, but not enough to condition it by controlling the humidity level.
Most boilers used in the kiln drying industry are rated to provide steam at 15 lbs. per square inch. A general rule is that five BTU of steam energy are needed per board foot per hour for conditioning. Typically, in a seven day drying cycle the first two days dry off the surface water (or ice). After that, for the next four to five days, moisture is added to the kiln to prevent the checking. As an example, in a 30,000 board foot kiln, the amount of steam required at 5 BTU/board foot times 30,000 board feet equals 150,000 BTU per hour, or approximately five horsepower continuously.
Another approach is to use high-pressure steam to inject atomized water into the kiln. This method is expensive and requires energy for the boiler to produce or atomize the moisture, while keeping the dry bulb temperature from dropping.
A variety of approaches have been suggested for vaporizing liquids. For example, U.S. Pat. No. 4,604,109 by Koslow discloses an apparatus for separating contaminants from a fluid using a vacuum chamber and at least one rotating plate disposed within the chamber. The contaminated fluid is flung off the rotating disk in the form of a fine mist of droplets from which the contaminants evaporate. The purified droplets are then collected.
U.S. Pat. No. 4,833,895 to Johnson discloses a spin disk evaporator including a pan suspended from a cage enclosing a motor rotating a spin disk and a fan for moving an air stream across the pan and outward for the evaporator. The disk includes a cone-shaped tip immersed in water held in a sump portion of the pan. Rotation of the disk causes a film of water to be picked up by the tip and moved across the water transfer surface in a circumferential wall of the disk. Water collected on the inner surface of the wall separates and moves through grooves from the inner surface of the wall across the end of the wall to the outer surface. Water exits from the grooves and a circular head and is dispersed into the air stream as extremely fine water particles.
The approaches outlined above do not provide an apparatus or method for producing large quantities of water vapor in an inexpensive and efficient manner. Further, these approaches do not provide any suggestion of the manner in which water vapor can be produced efficiently and inexpensively for use in a wood drying kiln.
It is an object of the invention to provide an apparatus and a method to readily generate water vapor efficiently and inexpensively.
It is another object of the invention to use the generated water vapor to control the humidity in an apparatus such as a wood drying kiln.
It is a further object of the invention to prevent the occurrence of undesirable defects in wood such as checking, cracking, crazing or warping, by inexpensively and efficiently controlling the humidity in a wood drying kiln.
The invention is directed to an apparatus for generating water vapor comprising at least one hydrophilic disk; a water supply structure for supplying water so that the water is deposited on the at least one disk; and means for rotating at least one disk at a speed sufficiently high so that water, which is deposited thereon is caused to leave at least one disk in the form of a mist or aerosol of very small water particles which evaporate to generate the water vapor (humidity).
A disk in accordance with the invention may comprise a water absorbing fabric affixed to the disk. An adhesive layer, such as an epoxy, may be disposed between the disk and the fabric for bonding the fabric to the disk. The fabric may be one of a woven and a non-woven fabric, and may be comprised of a material selected from the group consisting of a polyester, a cotton and a cellulose. Alternatively, a layer of a phosphate may be disposed on the disk so as to render the disk hydrophilic.
The disk or disks may be rotated by an electric motor, a hydraulic motor or a pneumatic motor. The disk or disks may be directly attached to the shaft of the motor. The apparatus may be combined with a wood drying kiln, the apparatus being placed within the kiln to provide a source of water vapor as the temperature of the kiln is controlled to dry wood placed in the kiln.
The invention is also directed to a method for providing water vapor, comprising rotating at least one hydrophilic disk at speeds sufficiently high to fling a fine mist of water particles from the disk; and providing water to said at least one disk.
Another aspect of the invention is a method of drying wood in a kiln, comprising heating air in the kiln; adding water vapor to the air by providing water to at least one hydrophilic disk, rotating the disk or disks at speeds sufficiently high to generate a mist of fine water particles, and circulating the air to which water vapor has been added to dry the wood. This method may be thought of as lumber conditioning; that is the process used to relieve stresses that build up in lumber as a result of drying of the lumber.